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Multiplexer
Background A multiplexer (or mux) is a device that selects one of several analog or digital input signals and forwards the selected input into a single line. This data selector/multiplexer contains full on-chip decoding to select the desired data source. The 74LS151 was the chip that was used. Its data sheet can be accessed on the References pages The 74LS151 selects one-of-eight data sources. The 74LS151 has a strobe input which must be at a low logic level to enable the device. A high level at the strobe forces the W output HIGH, and the Y output LOW. W and Y are the outputs, and W = NOT Y. By using these two pins, the chip can be used as an active low or active high device System Description The role of the multiplexer in the circuit is to choose which bit from which register is passed on to the 7 segment display board. This allows 1 set of bits representing the minutes or hours of the clock to be displayed on the board. This schematic includes the decoder, counter and GAL chip that is used to drive the select pins and the segments on the displays. The decoder is in charge of selecting which segment to turn on. The GAL interprets the bit pattern sent from the multiplexers and lights up the segments. The counter drives the selection pins for the decoder and multiplexers. The Multiplexers used were 74LS151, the counter used was a 74LS93, the decoder used was the 72LS138 and a 74LS08 (AND GATE chip) were used. Testing 1) Connect circuit and turn on power. 2) Observe numbers on test points and compare them to the ones that are expected. 3) Toggle switch to see if any change occurs. 4) If proper switching does not occur, begin to measure point voltages, and check for any obvious wiring errors. Trouble Shooting Upon further inspection of circuit, there were several wiring errors, which were corrected. Table 1. Troubleshooting table for trying to get the values displayed. ^ _ denotes no output. * It was determined that the outputs from the decoder were in the wrong spot. Table 2. Troubleshooting table trying to get the right flashing pattern ^ _ denotes no output. This table clearly shows that the output from the decoder was inverted. We then realized the decoder had active low outputs and had to be inverted, leading to the inclusion of the 74LS04 inverter chip. Conclusion After minor adjustments, the circuit operates as designed. We could potentially make the circuit neater, as in its current state the long wires are quite obstructive to viewing the circuit. We could also make changes to the 7 segment board. Specifically we could change the wires from just green to colour coded. The circuit functioned quite quickly after it was finished being built, so in further design and build processes we should not alter too much. Due to the miniscule amount of trouble encountered while troubleshooting the circuit, there was not much room for result analysis. All result analysis was done quickly and the solutions were found quickly.